Insights Into the Nature of the Transition Zone from Physically Constrained

Insights into the nature of the transition zone from SPECIAL FEATURE physically constrained inversion of long-period seismic data Fabio Cammarano* and Barbara Romanowicz Berkeley Seismological Laboratory, University of California, 215 McCone Hall, Berkeley, CA 94720 Edited by Ho-kwang Mao, Carnegie Institution of Washington, Washington, DC, and approved February 7, 2007 (received for review January 4, 2007) Imposing a thermal and compositional significance to the outcome of that a seismic model is not required to have a meaning in terms the inversion of seismic data facilitates their interpretation. Using of these physical parameters. long-period seismic waveforms and an inversion approach that in- Typically, the interpretation of a given seismic data set is cludes constraints from mineral physics, we find that lateral variations performed in two steps. First, a seismic model is constructed that of temperature can explain a large part of the data in the upper fits the data satisfactorily. Second, the model is interpreted in mantle. The additional compositional signature of cratons emerges in terms of temperature and composition based on the knowledge the global model as well. Above 300 km, we obtain seismic geotherms of the elastic and anelastic properties of mantle minerals plus that span the range of expected temperatures in various tectonic constraints on plausible composition and temperature ranges regions. Absolute velocities and gradients with depth are well con- from geochemistry [i.e., the signature of outcropping rocks strained by the seismic data throughout the upper mantle, except (orogenic peridotites) and mantle inclusions (xenoliths and near discontinuities. The seismic data are consistent with a slower xenocrysts)], heat flow measurements at the surface, and con- transition zone and an overall faster shallow upper mantle, which is ditions for melting mantle materials. The second part of the not compatible with a homogenous dry pyrolite composition. A process is commonly regarded as critical for interpretation. gradual enrichment with depth in a garnet-rich component helps to Together with the trade-off between temperature and compo- reduce the observed discrepancies. A hydrated transition zone would sition, other factors should be considered. Temperature- help to lower the velocities in the transition zone, but it does not dependent anelasticity implies nonlinearity of the partial deriv- explain the seismic structure above it. atives with temperature in the upper mantle (1). Consequently, absolute velocities are required for a correct interpretation. The mantle composition ͉ seismology ͉ mineral physics presence of mineralogical phase transitions further complicates the interpretation of the transition zone structure (14, 16). Finally, the large uncertainties that still characterize the elastic he thermal state and composition of Earth’s upper mantle and, even more, anelastic parameters derived by mineral physics and transition zone dictate its dynamics from microscale GEOPHYSICS T (see refs. 14, 17, and 18) hamper a precise interpretation. (e.g., creep mechanisms and earthquakes) to macroscale (e.g., Instead, it is often forgotten that the seismic models are already modality of mantle convection and plate tectonics). The knowl- an interpretation of the data. The seismic models are not unique, edge of these fundamental physical parameters and their 3D and they depend on the parametrization, distribution, quality, variations in Earth’s deep interior is indirect and relies entirely and type of data used. More importantly, a given best-fit seismic on the interpretation of geophysical data, based on insights from model does not necessarily correspond to a physical model. For theoretical and experimental mineral physics. Among these data, example, the previous work of Cammarano et al. (14) has pointed seismological observations constitute a main source of informa- out the difficulties of interpreting seismic reference models tion. Seismic waves record information about the elastic (and [Preliminary Reference Earth Model (PREM) (19) and AK135 anelastic) structure of Earth. Long-period seismic data provide (20)] in terms of temperature and composition. The same the most comprehensive global constraints on upper mantle problems are propagated to 3D tomography models that are shear velocity structure. Fundamental-mode surface waves are obtained by perturbing a starting average model. mostly sensitive to the uppermost mantle structure, and includ- To overcome this problem, it is indeed important to test a ing overtones provides resolution in the transition zone. physical hypothesis directly against the seismic data (as in refs. 21 Lateral temperature variations in the Earth have been inferred and 22). The general idea is to use the mineral physics information since the first tomographic studies began to reconstruct 3D already at an early stage of the seismic data processing, hence seismic velocity structure (1, 2). However, despite the ever- imposing a physical significance to the seismic tomography model. improving resolution of seismic velocity models and a general Here we invert long-period seismic waveforms, which are mainly agreement of different models on at least the large-scale struc- sensitive to shear velocity, with respect to a physical reference ture (e.g., refs. 3–6), interpretation is still challenging. The few model instead of a standard 1D seismic reference model (e.g., quantitative interpretation efforts made to date have shown that PREM). Velocity variations will thus correspond to thermal (or much of the seismic heterogeneity in the uppermost mantle is compositional) variations and a consistent 3D density and VP probably thermal in origin (7, 8), but it is likely that there is a compositional contribution under cratons (9, 10) and that fluids influence the very low velocities in the mantle wedge above Author contributions: F.C. and B.R. designed research; F.C. and B.R. performed research; subduction zones (8). By adding constraints from gravity or F.C. and B.R. analyzed data; and F.C. and B.R. wrote the paper. geoid data, joint inversions for seismic velocity and density have The authors declare no conflict of interest. been performed because of the better chance to isolate thermal This article is a PNAS Direct Submission. and compositional effects (11–13). Whereas temperature sensi- Abbreviations: PREM, Preliminary Reference Earth Model; PREF, Physical Reference. tivity dominates for seismic velocities, especially at shallow *To whom correspondence should be addressed. E-mail: [email protected]. depths (7, 14, 15), density also strongly varies with composition. This article contains supporting information online at www.pnas.org/cgi/content/full/ Besides the trade-off between temperature and composition, 0608075104/DC1. an important issue for seismic interpretation concerns the fact © 2007 by The National Academy of Sciences of the USA www.pnas.org͞cgi͞doi͞10.1073͞pnas.0608075104 PNAS ͉ May 29, 2007 ͉ vol. 104 ͉ no. 22 ͉ 9139–9144 Downloaded by guest on September 27, 2021 structure may be determined. We start by assuming purely thermal Events with moment magnitude larger than six and stations at variations because, in the upper mantle, composition plays a epicentral distance between 15° and 165° were selected. The secondary role. We compare the thermal features constrained by original seismograms have been deconvolved for the instrument the seismic data against expected thermal variations and estimated response and filtered between 60 s and a variable maximum geotherms in various tectonic regions. The average velocity model period, typically between 220 s and 1 h, which was chosen extracted from the physically constrained tomography provides according to the event magnitude. Wave-packets for both fun- insights into the nature of the transition zone. To assess the damental and overtones were then extracted from the seismo- uncertainties in the thermal model and estimate how well average grams based on the comparison of the observed trace with the velocity and velocity gradient with depth are constrained by long- PREM synthetics (5). The PREM represents very well the period seismic data in the upper mantle, we perform a series of average seismic structure of Earth. The bounds of the selection inversions starting with various models. criteria are large enough to make the procedure appropriate also for the alternative average upper mantle structure based on a Seismic Procedure physical model. Minor and major arc paths were selected for our The Starting Physical Model. Because seismic inversion is based on inversion. In total, the data set amounts to 39,829 fundamental perturbation theory, to be used as a reference model, the starting waveforms and 59,831 overtones. The windowing scheme not physical model must have an overall good fit to seismic data, only reduced the computation time of the inversion, but it comparable to the fit of radially symmetric seismic models (e.g., allowed the application of appropriate weights to different PREM). We tested our inversion by using some of the Physical phases. In our case, we assigned a double weight to the overtones Reference (PREF) models from Cammarano et al. (22). These are compared with the dominant fundamental modes. Noise and isotropic models with the same thermal and compositional struc- path redundancy were also considered in the weighting scheme ture, respectively 60-million-year-old oceanic geotherm in the litho- as described by Li and Romanowicz

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